Impact of Maternal Postpartum Tetanus and Diphtheria Toxoids and Acellular Pertussis Immunization on Infant Pertussis Infection

MAJOR ARTICLE Impact of Maternal Postpartum Tetanus and Diphtheria Toxoids and Acellular Pertussis Immunization on Infant Pertussis Infection Luis A. Castagnini,1 C. Mary Healy,1,3 Marcia A. Rench,1 Susan H. Wootton,4 Flor M. Munoz,1,2 and Carol J. Baker1,2,3 1Department of Pediatrics and 2Department Molecular Virology and Microbiology, Baylor College of Medicine; 3Center for Vaccine Awareness and Research, Texas Children's Hospital; and 4Department of Pediatrics, University of Texas Health Science Center at Houston (See the Editorial Commentary by Libster and Edwards, on pages 85–7.) Background. Mothers often are the source of pertussis illness in young infants. The Centers for Disease Control and Prevention recommend tetanus and diphtheria toxoids and acellular pertussis (Tdap) vaccine for postpartum women before hospital discharge. In January 2008, this recommendation was implemented in a predominantly Hispanic, medically underserved population at Ben Taub General Hospital (BTGH) in Houston (hereafter the intervention population). Methods. A cross-sectional study compared preintervention (July 2000 through December 2007) and postintervention (January 2008 through May 2009) periods. Pertussis diagnosis was determined using International Classification of Diseases, Ninth Revision (ICD-9) codes and microbiology reports from 4 major children’s hospitals in Houston. Only those infants #6 months of age with laboratory-confirmed pertussis illness were included. The proportions of pertussis-infected infants born at BTGH in the pre- and postintervention periods were compared. Results. Of 514 infants with pertussis, 378 (73.5%) were identified during preintervention and 136 (26.5%) during postintervention years. These groups were similar in age (mean, 79.3 vs 72 days; P 5 .08), sex (males, 55% vs 52%; P 5 .48), length of hospitalization (mean, 9.7 vs 10.7 days; P 5 .62), mortality (2 deaths each; P 5 .29) and hospital of pertussis diagnosis. After adjustment for age, sex, and ethnicity, the proportions of pertussis-infected infants born at BTGH and potentially protected through maternal postpartum Tdap immunization were similar for the 2 periods (6.9% vs 8.8%; odds ratio, 1.06; 95% confidence interval, 0.5–2.2; P 5 .87). Conclusions. Immunizing only postpartum mothers with Tdap vaccine did not reduce pertussis illness in infants #6 months of age. Efforts should be directed at immunizing all household and key contacts of newborns with Tdap, not just mothers. Despite high infant immunizations rates, pertussis is the only vaccine-preventable disease in the United States for which the incidence reached a nadir in the late 1970s, subsequently increased, and remains high. Although the greatest number of pertussis cases reported to the Centers for Disease Control and Prevention (CDC) Received 30 March 2011; accepted 17 August 2011; electronically published 10 November 2011. Correspondence: Carol J. Baker, MD, Department of Pediatrics, Baylor College of Medicine, 1102 Bates St, Ste 1120, Houston, TX 77030 (). Clinical Infectious Diseases 2012;54(1):78–84 Ó The Author 2011. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: . DOI: 10.1093/cid/cir765 78 d CID 2012:54 (1 January) d Castagnini et al annually occur in adolescents and young adults, the highest age-specific attack rate is among infants aged ,6 months, in whom rates are up to 20-fold higher than in other age groups [1, 2]. The highest rate of pertussisassociated complications, hospitalizations, and deaths also occurs in young infants who have not yet completed their 3-dose pertussis immunization series at 6 months of age [2–6]. For reasons that are poorly understood, Hispanic infants have substantially higher rates of pertussisassociated complications and death than infants of other ethnicities [2, 5]. The 2010 pertussis epidemic in California, which afflicted more persons than in the previous 65 years, powerfully illustrates the vulnerability of young infants and the disparity in pertussis rates for Hispanic infants [7]. The increase in pertussis cases in the United States in the latter 20th and early 21st centuries is probably multifactorial. Increased physician awareness, improved surveillance, more accurate reporting of cases, and greater use of more sensitive diagnostic methods, such as polymerase chain reaction (PCR), each probably contributes. One important explanation for the increase in pertussis cases is waning immunity 5–8 years after children receive their final diphtheria and tetanus toxoids and acellular pertussis (DTaP) vaccine dose at age 4–6 years, rendering adolescents and adults susceptible to pertussis [8–11]. Once infected, adolescents and adults become important transmitters of infection to young infants. In up to 75% of infant pertussis cases, the source of infection is another household member, who may have no or only mild pertussis symptoms. Most often the source is a mother with undiagnosed pertussis infection (an estimated 33% of cases) [12–18]. In 2006 the Advisory Committee on Immunization Practices to the CDC recommended that postpartum women and adolescent and adult household contacts of newborn infants receive immunization with tetanus and diphtheria toxoids and acellular pertussis vaccine (Tdap) before hospital discharge (‘‘cocooning’’) [8]. The aim of cocooning is to interrupt transmission to young infants before they are able to complete the primary DTaP vaccine series, with the hope of reducing their pertussis-associated morbidity and mortality. However, apart from reports from 1 birthing center, 1 neonatal intensive care unit, and some pediatric office settings, cocooning has not been widely implemented [19–21]. Computer simulation models predict a strong indirect effect of cocooning, estimating that immunizing mothers and household members could result in a 70% reduction in the incidence of pertussis in infants 0–3 months of age [22], but no studies have directly evaluated the impact of cocooning on infant pertussis illness. Moreover, no models have addressed the impact of immunizing only postpartum mothers as a single intervention. The objective of our study was to evaluate the impact of routine maternal postpartum Tdap immunization in preventing pertussis illness in infants #6 months of age. METHODS Study Design and Subjects We performed a cross-sectional study comparing 2 time intervals: preintervention (July 2000 through December 2007) and postintervention (January 2008 through May 2009). The intervention was a routine standing order for maternal postpartum Tdap immunization at Ben Taub General Hospital (BTGH), Houston, Texas. The majority of pertussis cases in infants in Houston (and a substantial proportion of cases in infants who are not hospitalized) are diagnosed and treated in 1 of 4 Texas Medical Center hospitals: Texas Children’s Hospital, Children’s Memorial Hermann (...truncated)